"Soda" pulping is a process using only sodium hydroxide as the chemical reactant. On wood chips, this process normally uses high temperatures (up to 160oC) and high pressures (up to 10 atmospheres) to dissolve lignin. However, some non-wood plants can be adequately delignified using a boiling solution of sodium hydroxide, provided some mechanical pulping follows the chemical pulping.
The Moisture Content of Plant Materials
All plant materials are "hygroscopic," meaning that they absorb water in equilibrium with their environment. For proper dosing of chemicals, the actual content of moisture-free fiber in raw material must be determined. The term "consistency" is used in the paper industry to describe the dry solids content (by mass) of a fibrous material. The total mass of a sample of raw material, multiplied by the consistency, equals the moisture-free mass, also call the "oven dry" mass.
Most plant material, if allowed to dry in open air for several days, will achieve a consistency of 90%. Conversely, they achieve moisture content of 10%. The term "air-dry weight" is normally used to refer to the as-is mass of the material or the mass obtained when the dried material is placed on a balance.
The following formula is used to convert from air-dry to oven-dry weight:
Oven-dry mass, grams = (Air-dry mass, grams) x 90%
Procedure
Use the following procedure to conduct soda pulping on the raw material of your choice:
Using a balance, mass out 50 air-dry grams of raw material. Calculate the oven-dry weight of this 50-g using the formula above.
For a soda process, the following process specifications will be used:
Chemical concentration: NaOH, 18% by mass in water
Temperature: 100oC (boiling)
Time: 30 minutes
Prepare an 18% sodium hydroxide solution by pouring 700 ml of water into a 2000-ml beaker. Weigh out 154 grams of the sodium hydroxide and add it to the beaker containing the water. Stir gently until the sodium hydroxide is dissolved. Note: the liquid will become warm due to the "heat of solution," a term used to describe the release to energy when some solids are dissolved.
Place the massed raw material into the 2000-ml beaker along with the sodium hydroxide solution. Some material may try to float; it will sink once the heating process starts. It may be necessary to take a glass stirring rod and push some pieces of material down off the sides of the beaker.
Place the beaker on a hotplate and turn to the "high" setting. Monitor the beaker at all times. Stir the contents occasionally to promote uniform heating. When the mixture starts to boil, reduce the heat to the "medium" setting. The goal is to establish a slow, uniform boil.
Once the boil is established, start timing the reaction. Permit the boil to continue for 30 minutes.
NOTE: as the boil progresses, the liquid will turn dark brown or even black. This behavior is due to the dissolution of lignin and other components.
At the end of the boil, turn off the hot plate and set the beaker aside to cool for 15-30 minutes.
Using a pair of long tweezers or a glass rod, remove a small portion of the boiled material fiber and place it on a paper towel. Notice that, while made softer by the boil, the material is still not in the form of small fibers. It will be necessary to provide some mechanical action to the cooked material to "fiberize" it into pulp fibers.
The dark-colored liquid mixed in with the boiled material is known in the pulp and paper industry as "black liquor." It must be separated from the solid material before fiberizing and washing. Turn on the cold tap water at the sink to a moderate level. Place the fine-mesh colander or strainer into the sink. Using a gloved hand, slowly pour the contents of the beaker into the strainer, allowing the liquid to drain into the sink. It may be necessary to use a glass or metal rod to get all of the solid material out of the flask. Let the water run into the sink for several minutes after draining is complete to provide a good flush of the chemicals.
When the boiled material has finished draining, transfer it with a gloved hand into the blender container. Add 600 ml (600 grams) of cold tap water. Place the container cap on securely and hold down with your hand. Turn the blender on its lowest setting and permit to blend for 5 minutes.
After fiberization, the material may now be called "pulp." In its unbleached form, pulp is often referred to as "brownstock."
Stop the blender. Wash the pulp by placing 1 paper towel in the Buchner filtering funnel. Pre-wet the towel to make it conform to the shape of the funnel. Place the funnel into the filter flask, then connect the flask to the aspirator or other vacuum source. Turn on the vacuum. Slowly pour the contents of the blender container into the funnel. Liquid will start leaving the pulp immediately. When the liquid level in the filter flask reaches a point near the vacuum take-off, stop pouring into the funnel. Turn off the vacuum and remove the funnel, being careful not to dump out the contents. Turn on the sink tap water to a moderate level, then pour the flask contents into the sink. Replace the funnel, turn the vacuum back on, and resume pouring in material from the blender container.
When all the material has been poured form the blender container, allow all the liquid to be pulled from the pulp, forming a thick mat. Fill the blender container with hot tap water to the 1-liter mark. Pour this water on top of the pulp mat in the funnel slowly, allowing the vacuum to pull it through. Turn off the vacuum and discard liquid as necessary.
Repeat step 13 using another liter of hot tap water. The pulp should now be thoroughly washed.
This type of washing, which uses a vacuum to pull a cleaner liquid through a pulp mat and displace the "dirtier" liquid, is called "displacement washing." It is a very efficient method of washing and is used extensively in the industry.
Allow the pulp mat to drain until no liquid is noted coming out of the funnel. The mat will be slightly mushy to the touch. Turn off the vacuum and remove the funnel for the flask. Remove the paper towel for the funnel and lay flat. Separate the pulp mat from the paper towel, rolling the pulp ball across the towel to capture remaining fibers.
Take a small piece of the pulp mass and place it into a clean 1000-ml flask. Add 400-500 ml of water, cover with your hand, and shake vigorously to disperse the pulp. Hold the flask up to the light and you will see the liberated fibers suspended in water. You may also see bundles of poorly-separated fibers, know as "shives." These bundles are normally separated form the fibers in a process call "screening." They will hopefully be reduced to fibers during the bleaching step below.
Place the wet pulp in a sealed plastic bag to keep it form drying out. Label the bag with the raw material name or type, the pulping process used (soda), the chemical charge (18% NaOH).